Ask the Astronomer Q&A

Tag: "satellite"

How long does it take to go to Mars?

The length of time it takes to go to Mars depends on how fast you are going and how far away Mars is from Earth. This distance changes due to the constant motion of Mars and Earth around the Sun. The average distance between Earth and Mars is about 46.5 million miles. If you traveled at freeway speeds, 60 miles per hour, it would take 88.5 years! If you are travelling at the speed of light (a speed we are incapable of achieving), it would take about 4 minutes to reach Mars. The fastest spacecraft that was ever launched into space, New Horizons, passed Mars in about 40 days on its way to Pluto. An average mission to Mars takes about 6 months to reach the red planet.

Is it possible for a satellite to maintain an orbit on the far side of the Moon (always hidden from Earth), given the complicating effects of Earth's gravity? If it is possible, what would the semi-major axis be?

No, it would not be possible for a satellite to orbit completely hidden from the Earth. To keep a satellite on the far side of the moon, it would have to orbit the Earth, not the Moon, every 28 days. Using Kepler’s law of planetary orbits, we can find the semi-major axis of the object with this equation (p^2/(a^3). Where a is the semi-major axis and p is the period. We know we want the period to be the same as the moon’s, 28 days. Which means that the semi-major axis would be equal to the moon’s semi-major axis. So the only way for the satellite to remain on the far side of the moon, is to place it on the moon itself. It is possible for the object to share the moon’s orbit (by trailing behind or ahead of the moon), but then we would be able to see it.

The Lunar Reconnaissance Orbiter (LRO) scientists mentioned the Moon's craters that have ice in them are probably charged up by static electricity from interacting with the solar wind. Is there a way to measure the voltage from some probe or other?

This might be a good one to try to send to the LRO scientists because this will be depend on the amount of ice, as well as electromagnetic field strength caused by the solar wind.
Unfortunately, static electricity can’t be measured by probes unless the voltage carrying medium is grounded and discharged. For example, we can’t measure the static electric voltage building up in clouds. However, if we knew the amount of current that was transmitted to a lightning rod by the cloud, then we would be able to estimate the amount of static electricity carried in the cloud just before the lightning.

What equation would you use to determine the brightness of sunlight on a satellite when the distance from the sun is measured in AU?

There are two ways to calculate this:
1. Use Pogson’s equation to find brightness at desired AU (there is a special form of Pogson’s equation for this)
2. Imagine a great sphere with (R= satellite’s distance). Calculate the surface area of that sphere. Then divide the (total energy coming out of Sun’s surface) by the (area of the sphere). Now you have the amount of energy reaching up to the satellite’s unit surface. To obtain the flux (function of brightness), just multiply that (energy per unit area) by the area of detector (for human eye: 8mm wide circular lens). – note that brightness has no meaning by itself. Brightness is usually given on a scale based on a reference source. For example, stars’ magnitudes are often determined by comparing them with Star Vega (m = 0.0)

We can see other galaxies with ground and space-based telescopes, like the Hubble Space Telescope, some can even been seen with the unaided eye – like the Andromeda Galaxy. Hundreds of thousands of different galaxies have been seen. A famous image from Hubble, called the Ultra Deep Field Image, shows over 10,000 galaxies in one small part of the sky.